Thermoelectric Grain Boundary in Monolayer MoS2

Defects such as grain boundaries (GBs) fundamentally control thermal and electrical transport in two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, but the mechanism remains elusive. We have studied thermal and electrical transport across and along the GB within a monolayer of a prototypical TMDC, MoS2, using molecular dynamics simulation and first-principles quantum-mechanical calculation. We found the existence of an interfacial phase (or "interphase") within a few nanometers from the GB, which has anisotropic transport properties that are distinct from those of a perfect crystal. Namely, the GB interphase has reduced thermal conductivity across the GB. In stark contrast, the electrical conductivity of electron-doped MoS2 is enhanced in both directions, with higher conductivity across the GB. These results are understood in terms of the alignment of energy levels and spatial distribution of electronic wave functions around the GB. Such contrasting thermal and electrical transport properties of the GB interphase suggest a promising application of GB superlattices to thermoelectric power regeneration for sustainable future 2D electronics.